The present invention relates to improvements in fastener profile structures for plastic bags of the type which are reclosable.
More particularly, this type of bag is formed of thin plastic film of a suitable physical characteristic for containing products. Frequently, the bag will be of thin, lightweight tough material which is clear so that the contents can be seen with the bag fully sealed so that the contents are protected against dust and moisture. The bag is opened by being cut or torn at the top for access to the contents. If a reusable bag is desired, it has releasably interlocking fastener profiles provided on the inner facing surfaces near the top of the bag. These profiles permit the bag to be opened by pulling the top lips apart. The bag can be reclosed by applying a closing pressure along the profiles.
This type of bag is sufficiently inexpensive so that it does not add significantly to the cost of the product, and yet provides a container which is of use for carrying the product for some time thereafer or is reusable for other products. This makes the bag particularly valuable to the purchaser and increases the merchandisable attractiveness of the product.
To provide such an improved bag at a reduced cost, it has been discovered that certain fastener profiles can be carried on fastener strips which are laminated or attached by heat and pressure to the surface of a film which is then converted into a bag with the fastener strips located at the inside and near the top of the bag. With this construction, the strips can be formed of optimum material for the fastener profiles, and the bag can be made of material chosen independent of the requirements of the fastener strips. For example, a thin flexible plastic bag may be provided of a very clear plastic, and a more rigid slippery material can be used for the fastener strips of optimum physical characteristics for the interlocking profiles. Such profiles will interlock with a strength which equals the strength of the bag, and yet can be readily openable when desired by pulling apart the bag's top flanges of lips.
To be compatible with a flexible plastic bag, the fastener strips of this type must be capable of being inexpensively manufactured and must be available in a form so that they can be attached rapidly to a plurality of bags in an assembly line arrangement. If the fastener strips are expensive to make, or inconvenient to handle so that they are expensive to assemble, the advantage of a low cost plastic bag is defeated, and the commercial attractiveness of this type of product is lost.
It has heretofore been customary to form certain fastener strips by extruding them individually from a die which is shaped to form a planar web portion with a shaped profile on the surface. Also, this specific type of fastener consists of separate female and male profiles which interlock when pressed together, as illustrated in the attached drawings. The web portion is flat, and the profile is usually centrally located. In this type of extrusion, the resistance within the die will tend to pull unequally so that frequently there is a holding back of the web portion on one side or the other of the profile. This tends to cause cocking and distortion of the male or female profile and wrinkling of the fastener web. It will be apparent that it is critically important that the profile retain a perfect shape and retain the dimension which has been determined for it so that it will interlock with full strength with its mating profile.
The present invention extrudes a plurality of male and female fastener strips at least six in number, in a single extrusion, so that the web portions of the strip are each attached to each other, or in other words, form a tube. This eliminates the unequalness which is encountered with the extrusion of a single flat strip, and the pulling and web wrinkling which occur with the single strip are eliminated.
Another advantage obtained in extruding the plurality of strips joined together in a single tube is that the web portion of the strip can be made thinner. There is a minimum limit to the web thickness which can be extruded when a flat single strip fastener is made because the web will tear and not extrude if the opening gets too small, or if the fastener is drawn down too much in order to reduce its size. However, by extruding the plurality of strips in tube form, the web can be thinned down by inflation of the tube to stretch it to a thinness less than the die opening. This inflation will occur without reducing the size of the profile since inflation of the tube will only reduce the size of the web which supports the profile. In this manner a fastener strip can be made having a web of a thickness which is the same as the bag so that the web can be heat sealed to the surface of the bag without damage to the bag material. If an attempt is made to seal a fastener strip having a thick web to a thin bag, enough heat must be added to melt the plastic of the strip, and this heat usually will be too much for a thin bag film and will result in melting holes in the bag wall. With the fastener strip and bag film of substantially equal thickness, enough heat is added to soften the plastic of both the film and the strip an equal amount, and this accomplishes a satisfactory seal without resulting in holes in the bag film.
A still further advantage of the formation of the male and female profile strips in tube form is that a greater speed of extrusion can occur as contrasted with single flat strip extrusion of these profiles. Limitations are inherent in a single flat strip extrusion because the high speeds result in wrinkling and distortion which do not occur in tube extrusion.
A still further advantage of the present invention is that smaller male and female profiles can be made. Limitations in drawdown tend to limit the size of the profile in the extrusion of a flat single strip.
Also, in a flat extrusion process the more single profile strips that are made at one time, the more difficult the extrusion becomes since each additional profile strip merely multiplies the problems encountered with a single profile strip. A circular multiple profile joined profile strip, however, tends to balance the profiles against each other, thereby reducing the difficultly of the multiple profile extrusion.
It is accordingly an object of the invention to provide an improved mechanism and method for the production of male and female profile strips of the type illustrated for attaching to plastic film for conversion into bags wherein improved control and cooling are achieved for a resultant improved profile shape and size.
A further object of the invention is to provide an improved mechanism and method for providing male and female profile strips wherein a thinner web portion can be obtained and wherein wrinkling and distortion of the web and edges of the profile are eliminated.
A further object of the invention is to provide a method and mechanism for extruding male and female fastener profile strips at higher rates of speed than heretofore obtainable in a conventional single flat strip extrusion process.
It is accordingly an object of the present invention to provide an improved male and female fastener strip structure of the type illustrated by a more rapid less expensive procedure wherein the material is particularly easy to handle for use in assembly with film for conversion into bags and wherein the strips are laminated onto the surface of the film of the bags.
Another object of the invention is to provide an improved male and female fastener strip structure which does not have the distortions and other disadvantages which occur from nonuniform cooling.
Other objects and advantages, as well as equivalent structures and methods which are intended to be covered hereby, will become more apparent to those versed in the art with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings in which: